Fate and impact of Bay of Bengal rivers in an intermediate resolution ocean model

Abstract

Annually, the Bay of Bengal receives about 1500 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> of freshwater from the rivers along its boundary. The decrease in salinity caused by the dilution due to river water strengthens the vertical stratification, creates shallow mixed layers, and enables faster SST recovery. The high SST is a factor that promotes cloud formation, monsoon depressions and tropical cyclones over the northern Bay of Bengal. Therefore, understanding the role of river runoff in maintaining the high SST is an important problem, particularly with respect to short range weather forecast. Using a global configuration of the GFDL Modular Ocean Model 5 at a horizontal resolution of about 13 km and 42 vertical layers, we have investigated the impact of river runoff on the upper layer thermo-haline structure and circulation in the north Indian Ocean. Two numerical experiments, one with the river runoff into the ocean excluded and the other by including climatological monthly mean river runoff are carried out for a period of 40 years each, to determine the pathways of river water in the Bay of Bengal and into the Arabian Sea. During summer, the river water is confined in the northern Bay of Bengal, but during winter it flows southward along the western boundary into the Arabian Sea. We have identified that there is significant impact on the sea surface temperature in the northeastern Arabian Sea where the river water causes warming of the sea surface. The change in the northeastern Arabian Sea is linked to the northward advection of low salinity water by the West India Coastal Current (WICC). In the absence of Bay of Bengal rivers, the wintertime East India Coastal Current (EICC) and the WICC, which carry the low salinity river water are weaker. The northward extent of low salinity water along the west coast of India is restricted in the absence of river runoff and consequently, the salinity of the northeastern Arabian Sea tends to be slightly higher. The north eastern Arabian Sea is warmer in the presence of river runoff into the BAy of Bengal. This warming is attributed to the strengthening of the WICC and the northward advection of warmer water when the river runoff is present. The model simulations suggest that the Bay of Bengal rivers have implications to places both near the source region and far into the Arabian Sea where the coastal currents (EICC and WICC) carry the low salinity water.